This research focuses on the development of an ultrasound imaging system for real-time three-dimensional echocardiography. The heart is a geometrically complicated structure that is constantly moving within the chest. Traditional echocardiography provides images of the heart in a two-dimensional format. A successful diagnosis relies on the skill of the diagnostician to mentally transform these dynamic two-dimensional images into the complex three-dimensional cardiac structure. Although attempts at three-dimensional cardiac imaging based on the acquisition of multiple two-dimensional images have been made, the rapid and complex motion of the heart makes this approach difficult. The applicants proposed to solve this problem by developing a high speed ultrasound imaging system that can scan the entire heart in the time normally required to collect a single two-dimensional image. High speed imaging (1000 2-D images/s) will be obtained without sacrificing image quality by using a sparse synthetic aperture technique. Preliminary theoretical and experimental work demonstrates the feasibility of this approach. A prototype high speed scanner will be built and the performance of the scanner will be evaluated using test phantoms. The new approach to three-dimensional ultrasound imaging promises to provide an important tool for visualizing and quantifying cardiac anatomy and function.